Why aren't polygraph (lie detector tests) inadmissible in court?

They are illegal because their accuracy is very low, to the extent that they don't satisfy the standards of evidence required by most courts. Repeated scientific studies have not found that polygraphs are an accurate way of determining if someone is lying or not. The US government did a study on their accuracy a few years back (you can download the pdf at the link below) and there were several serious flaws in its usability, including: * Low general accuracy and high rates of false positives. * Accuracy could vary considerably depending on the individual being tested. * There was real possibility that someone with the appropriate know-how could fool the test. As such, polygraphs are not a reliable means of telling if someone is lying or not, and hence should not be used in court. As well as being unable to provide any evidence, their use could also mislead a jury into thinking someone's testimony is more or less truthful than it actually is.

because they are inherently unreliable. There are some operators that are more effective than others, but none of them approach anything reliable enough to bet someones freedom or fortune on.

they are unreliable and it's easy to beat them a psychopath or sociopath would beat one easily . even mistakenly believing what you say is true will fool it

Because ... while the basic practice of measuring various body readings might be scientific, trying to INTERPRET those readings as they relate to telling the truth or lies is not. In other words ... 10 people could look at the same readings and potentially come up with different conclusions. ALSO ... people without a conscience won't register as deceptive. AND ... people who are overly paranoid about lie detectors are more apt to register as deceptive. ALSO ... people can take various counter measures to polygraphs. ALL OF THIS combines to make them highly questionable tests!

Because Lie Detectors do not detect lies. They detect changes in your stress level. Generally a stress level change to a higher level is an indicator of a POSSIBLE Lie. But if you are nervous to begin with, even telling the truth will come out as a lie. If you are a sociopath with little to no empathy, you can lie through your teeth and remain completely calm (and according to the machine... truthful) If you honestly think what you are saying is the truth, even if you are wrong... it will come out as truthful. If you are asked a question that makes you think of something else that makes you nervous yet you are not worried about the actual question, a truthful answer will still detect as a lie. An example of that last one is if you were asked: "Did you cheat on your wife with the suspect?" Even if the answer is truthfully no. if you had cheated on your wife with someone else... a stress level change associated with that guilt could make the original question seem a lie. Basically it boils down to this. They are not foolproof. They can be beaten by those intentionally lying and they can give false readings of lies to those who are truthful. While not admissible in court, a DA can use lie detectors as a guide to determine whether a case has merit and whether to prosecute or not. Everything must be proven in court though without the lie detector. The exception is sex related crimes. A DA cannot force a sex crime victim to take a polygraph, he can request it though. If the victim agrees or disagrees, the DA CANNOT use it as a determination to take the case or not. Why? because it is near on impossible for the most truthful of rape victims to pass a poly. simply reliving the crime in their minds as they will during questioning will send their stress through the roof. results would be meaningless. Everyone is basically saying the same answer. so it is curious as to why every answer from he top down has one thumbs down until you get to "One Mans Opinion". Some jerks just vote down every answer but their own (and would vote themselves a thumbs up if they could). So I gave him a thumbs down and everyone else a thumbs up.

Because some people can fool the machine. The machine needs to be 100% accurate. Current polygrapghs are about 92% accurate.

You need to take a psychology class. They talk about this very thing. The reason polygraphs are not admissable in court is because they are not 100% reliable, unlike fingerprints and DNA, which are scientifically reliable. People can fool the the polygraph machine. Polygraph machines measure heart rates and stress level, which can be misleading because if you are taking a polygraph, you'll more than likely be nervous.

Many of the answers thus far have been spot on. I will say that DNA and fingerprints are not 100% reliable-- there is a small percentage of error rate but it is so minute that it is unlikely for them to be incorrect. I will add that, unlike the DNA and fingerprints which are objective and verifiable, the polygraph results are subjective. Not only do the graph readings depend on the stress level of the person taking the test, but it also depends on the interpretation of the results by the person administering the test. Studies found that polygraphers may be subconsciously influenced by suggestion. In a study where polygraphers were told in advance that a certain person was a suspect, most of the polygraphers claimed the person failed the test-- even though the person was actually telling the truth. According to what was shown on Mythbusters, even if someone passes by the standards of the registering on a graph, a polygrapher could still claim a person failed or passed based on what he/she observed of the person's body language. So it is possible for the graph itself to register no deceit but for the examiner to think that the examinee was prevaricating. Several factors can also affect the results: The skill of the polygrapher, the mood of the polygrapher, whether or not the polygrapher likes/dislikes someone's face/demeanor, how the examinee is feelilng-- being ill or tired can impact the results, and the examinee's ethnicity. Ethnic minorities are far more likely to be given failing results even when they are telling the truth. The most useful application of the polygraph is not in determining whether or not someone is lying, but in the interrogation process to fool someone in to suspending their fifth amendment rights and in some cases, intimidating them in to a confession. If a guilty person believes that polygraphs are accurate and that they will get caught, they might confess to avoid getting in to further trouble. Unfortunately, innocent people might be afraid that they will not be believed-- this more often is a problem after a polygraph is administered and the person is ruled to have failed. As far as I know, the police can not force a suspect to take a polygraph against his/her will. I will add that a person doesn't have to be a sociopath or psychopath to pass the polygraph exam while lying. I think that the sociopath myth is just something law enforcement perpetuates to try to assure people of the reliability of the polygraph in order to continue using it as an interrogation tool.

Because they are 85% accurate -- and that is assuming the person isnt deliberately trying to give the machine a false reading